STR testing measures relationship in a time frame of hundreds of years, making the distance to the MRCA (most recent common ancestor) sufficiently close to be genealogically useful.  And "useful" is an understatement.  STR testing is a powerful tool for determing whether people can or cannot have shared a recent common ancestor.

There is another form of Y-chromosome DNA testing called SNP testing (the acronym stands for "single nucleotide polymorphism" and is pronounced as a single word, "snip").  Results of SNP testing are expressed as + or - (positive or negative) to indicate the presence or absence of a particular mutation.  Each suspected mutation requires a separate test, which is one reason SNP testing can become more expensive than STR testing.  Results of a basic, "backbone" SNP test determine a person's haplogroup ("Hg" is an abbreviation for haplogroup), and additional testing can identify subgroups, called "subclades." 

The haplogroup is a measure of deep ancestry.  The MRCA may be thousands or tens of thousands of years in the past, which places the individual's ancestry in a paleoanthropological time frame.  Haplogroup determination is of practical use for the project because it broadly defines and separates the most distinct family groups; that is, families with no prospect whatsoever of a connection in "genealogical time," making it a useful way to divide up the project.  For the individual, knowing one's SNP subclade greatly enhances one's appreciation of history because the literature on the genetic history of humanity, correlated with historic and pre-historic events, is growing rapidly.

If the STR-based haplotype is reasonably common (or similar to one that is), it can be used to deduce the probable SNP haplogroup, making the expense of actual SNP testing optional.  On the other hand, a haplogroup deduction based on a rare or unique haplotype can be wrong (due to small sample size), so a SNP test is recommended in such cases — assuming the subject is interested in an accurate haplogroup determinaton.  I think knowing one's haplogroup is a fun thing to know about oneself, but it is not a requisite for full participation in this project, which is based on the results of STR testing. 

I1-Norse
The Norse variety of I1 is most common in southern Norway and Sweden, Denmark, and northern Germany.  Our eight Norse subjects consist of a SORENSEN whose ancestors immigrated from Denmark and settled in Cheyenne, WY; a CHRISTENSEN with an origin in København, who is a 12/12 match with a BENDTSEN of Ringkøbing Amt, DK; a GLAD with an origin in Aalborg Amt; a WESTERGARD whose surname was originally JENSEN (prior to immigration to the U.S.); a HENDRIKSEN with an origin in Maribo Amt; a NEWMAN (NYMANN) with an origin on the Island of Møn; and a BAKTOFT with an origin in Randers Amt.  One has a perfect 12-marker match with the modal haplotype of I1-N, so has hundreds of matches in the FTDNA and Ysearch databases (all in other surnames); your project admin recommends upgrading to more markers to get some separation from his less related kin.  The remaining seven have rare to uncommon haplotypes.  Except for the CHRISTENSEN and BENDTSEN, they are not only not closely related to each other, they have no high level matches in the online databases. 

I1-Norse-D
Like Norse, this variety is most common in southern Norway and Sweden, Denmark, and northern Germany, but leans toward Denmark.  This individual, an ANDERSEN with an origin in Odense Amt, has a handful of 12/12 matches in the FTDNA and Ysearch databases.  The characteristic marker of I1-N-D (DYS439=12) is what prevents him from having many more matches.

I1-ultraNorse
I1-ultraNorse is the most common I1 variety in Norway and is the third most common variety in Sweden and Denmark; it is uncommon southwards.  We have four members of Danish Demes who are ultraNorse:  one is variety I1-uN1, a MØRCK with an origin in Århus Amt; one is variety I1-uN1a, a JENSEN with an origin in Maribo Amt; one is variety I1-uN-13-15, a HANSEN who is native to and a resident of Ribe Amt, DK; and one is a JØRGENSEN of Århus Amt, whose variety I cannot determine, unless he tests more markers.  The four are not closely related to each other.

I1-AngloSaxon
Of five Anglo-Saxon individuals, one is a HANSEN (origin Sønderborg Amt), one is a JENSEN (origin Viborg Amt), one is a CHRISTENSEN (origin Viborg Amt), one is a JØRGENSEN (origin Odense Amt), and one is a NIELSEN (origin Tønder Amt).  The NIELSEN is variety I1-AS6, but I was unable to determine the exact variety of the other four, each of whom has one or more unusual values that makes it unclear to which I1-AS varieties he belongs.

I2a-Dinaric
This individual is a JØRGENSEN with an origin in Ålborg Amt, DK.  His 12-marker haplotype is unique, with no matches in any online database.  Your project admin recommends deep SNP testing and upgrading to more markers.

I2b1-Root
We have one member, a LARSEN with an origin in Fredericksborg Amt, DK, who has a rare haplotype, with no matches at any level in the FTDNA database.  While his haplotype resembles the "Root1" and "Root2" varieties of I1b2a, the fit is not tight with either one, which is a reflection of the rarity of his haplotype.  Deep SNP testing confirms he is I1b2a.

I2b1-Continental-1
We have two members who fit this variety.  One is an ANDERSEN with an origin in Præsto Amt.  His haplotype is a perfect fit for the key markers defining this variety.  Still, his haplotype has no full matches above 12 markers.  The other is an ERIKSEN with an origin in Århus Amt, who has a rare haplotype with no full or near matches in any online database.  His haplotype suggests he is the "Continental-1" variety of I1b2a, but he has two very unusual markers values, so he is not a tight fit with the modal haplotype. Deep SNP testing confirms both are I1b2a.

I2b1-Continental-2
This individual is a HAMMER with an origin in Viborg Amt, DK.  He is a perfect match for the key markers defining Nordtvedt's "Continental 2" variety of I1b2a.  Still, his full haplotype is rare, with only a handful of matches at 12 markers and no full matches at 25 or more markers.

R1a1
This individual has been deep SNP tested, proving he is R1a1.  He is a LARSEN with an origin in Hjørring Amt, DK.  His 12-marker haplotype is common, but he has no full matches at 25 or more markers.

R1b1b2 [old R1b1c]
Except for the LORENZEN father and son, who have an origin in Tønder Amt, none of the seven individuals in this group match any of the others.  The other five are:  a HAUSO with an origin in Tønder Amt, a JENSEN with an origin in Vejle Amt, an ERIKSEN with an origin in Svendborg Amt, a SORENSEN
 

R1b1b2 — WAMH [old R1b1c — WAMH]
Six members have been designated WAMH by FTDNA meaning each has one of the four most common 12-marker haplotypes in western Europe.  One is a CHRISTENSEN whose origin is DK; one is a JAKOBSEN with an origin in Frederiksborg Amt; one is a JENSEN with an origin in Maribo Amt; two are VICKs with an origin in Præstø Amt; and one is a JØRGENSEN with an origin in Præstø Amt.  This last has the most common 9-marker haplotype in Denmark!  Not surprisingly, at 12-markers each has hundreds of perfect matches, but those matches fall away at 25 or more markers (except for the VICK father and son).

R1b1b2g [old R1b1c9]
The two MATTHIESEN half-brothers, whose ancestor originated in Tønder Amt, have been deep SNP tested are are determined to be R1b1b2g (i.e., "root" R1b1c9).  They have undergone Kittler test revealing they are hi-lo at DYS385ab, which is the modal order for R1b.  Other than with each other, they have no full or near matches with anyone else.

The mitochondria in our cells perform the critical function of metabolism, using oxygen to burn fuel to supply us with energy.  We would quickly die without them (e.g., cyanide acts by disabling them), so nature doesn't tinker much with these genes.  But there are two sections of the mtDNA strand that appear to have no function, so these sections are subject to retained mutations.  These sections are known as "hypervariable regions" and are named HVR1 and HVR2.  Without the two hypervariable regions, there wouldn't be enough mutation in the mtDNA molecule to be of much use in sleuthing out our ancestry.  As it is, even the hypervariable regions do not mutate frequently enough to be of use in short term (i.e., genealogical) time frames.  The mutations are extremely useful, however, in revealing our deep ancestry in a paleoanthropological time frame.

The first individual to have their mtDNA sequenced had it done experimentally in 1981, and their sequence has arbitrarily been made the standard against which all other mtDNA sequences are now compared.  This standard is called the "Cambridge Reference Sequence" (CRS); and, today, mtDNA test results are expressed as differences from the CRS.

Fortuitously, the CRS is also the most common mtDNA sequence in Europe (which, if you think about it, was statistically the most likely one to have been randomly chosen among the scientists in a laboratory in western Europe).  And while the CRS is the most common mtDNA sequence in western Europe, it is not the most ancestral one that has since been determined.  In a hierarchical (evolutionary) "tree" of mtDNA haplogroups, the CRS turns out to be Haplogroup H, popularly known as "Clan Helena."

FamilyTreeDNA offers two levels of basic mtDNA testing:  either HVR1 only or HVR1 + HVR2.  Because Haplogroup H is so common, FTDNA also offers a Haplogroup H "refinement" test (mt-H) that will place an individual in one of the currently known Haplogroup H subclades. The mt-H test analyzes a different suite of markers than either the HVR1 or HVR2 tests, so the tests are not redundant, but independent.  Your Haplogroup H subclade is a much broader category than your combined HVR1+HVR2 mutations.  So, if finances dictate taking only one test or the other and/or your aim is to find genetic matches, as opposed to musing on your deep ancestral origins, your project admin recommends taking the HVR2 test in preference to the mt-H test.  And, of course, as the ultimate refinement, FTDNA offers full sequencing of the entire mtDNA strand!

It is possible, though not common, to match the CRS at HVR1 and still not be Haplogroup H.  One needs to match the CRS at both HVR1 and HVR2 to be certain of being Haplogroup H.  So, if you have tested only HVR1 and match the CRS, it's recommended to also test HVR2.

Subclade H*
The asterisk indicates our subject has taken the mtDNA-H-refine test and possesses none of the mutations that would place her in one of the "downstream" H subclades.  In other words, she is "root" or "ancestral" Clan Helena.  She is a matrilineal descendant of Kirsten Eriksdatter of Skanderborg Amt.  Hers is a common haplotype, with hundreds of full and near matches in the FTDNA and mitoSearch databases, even with both HVR1 and HVR2 tested.

Subclade H1a
This individual descends from Wilhelmina (HANSEN) RASMUSSEN of København.  She has a common haplotype, with over 50 matches in the FTDNA database, even with her combined HVR1+HVR2 results.

Subclade H2a
We have a female descendant of Charlotte (BRUHN) HOLM of Haderslev Amt, whose mtDNA-H-refine test shows her to be subclade H2a.  She has a unique HVR1+HVR2 haplotype (no full match in either the FTDNA or mitoSearch databases).

Subclade H2b
Two project members are H2b, the same subclade as the CRS (Cambridge Reference Sequence).  One is the male LORENZEN mentioned above under Y-DNA test results, who also has Danish ancestry on his female line, his earliest known ancestor being Inge Jung Jensen of the Island of Föhr, Nordfriesland.  He has tested only HVR1, which shows no differences from the CRS.  He needs to test HVR2 to get any meaningful separation from distant kin because, given that he possesses the most common HVR1 haplotype in Europe, he understandably has hundreds of full matches in the online mtDNA databases.  The other H2b is a descendant of Olga Marie NIELSEN of København.  His combined HVR1+HVR2 haplotype is rare, with only one match in the FTDNA database.

Subclade H4a
This individual descends from a PETERSEN of København.  His haplotype is common, with over a thousand matches in HVR1 and some in HVR1+HVR2.  The ancestral origin of his high-level  H4a matches is entirely Danish.

Subclade H5a1
This subject's patrilineal ancestor is English, so he is not participating in the Y-DNA portion of the project; and while his mother's ancestry appears to be predominantly German, his key female ancestor — his mother's matrilineal ancestor — was Anne Elisabeth Kirstine Madsdatter, born in København, meaning his mtDNA fits our criterion for having a Danish origin.  He has tested both HVR1 and HVR2; and, despite being Haplogroup H, he has a rare haplotype with only one full match in either the FTDNA or mitoSearch databases.  His mtDNA-H-refine test shows him to be subclade H5a1.

Subclade H10
This individual descends from Johanne of Velling, DK.  Her haplotype is rare, and she had no matches in either the FTDNA or mitoSearch databases.

Subclade Unknown
We have three individuals who have not taken the H-refine test to determine their subclade; however even just their HVR results indicate they are not closely related, either to each other or to anyone else currently in the project.  One is a descendant of Kirsten Povelsen of Århus Amt who was tested at another lab and was designated Haplogroup H.  His haplotype is rare, even though only HVR1 was tested.  Another is a native of Denmark, whose maternal grandmother was a CHRISTENSEN, also born in Denmark.  While her HVR1 mutation is extremely common, giving her thousands of matches, her combined HVR1+HVR2 results give her just a few dozen matches.  The third descends from Inge Laursdatter of Haderslev Amt.  He has no matches, though he differs from the H1a member by just one mutation.  Your project admin recommends all three of these individuals take the H-refine test to determine their Haplogroup H subclade.

Haplogroup J arose in the Near East approximately 50,000 years ago.  It spread across Europe along with the spread of agriculture some 10,000 years ago.  The root lineage, J*, is found thinly throughout Europe.  We have one member who is J*.

Haplogroup K is a relatively old group, derived from Haplogroup R.  Given its age, it is widespread in Eurasia, the Near East, and northern Africa.  About 6% of Europeans are Haplogroup K, and about 60% of those are K1a.

Subclade K1c or K1c1
These two subclades can only be separated by mutations in the coding region of the mtDNA molecule; their mutations in the HyperVariable regions are the same.  This individual's earliest known matrilineal ancestor was Maren Anderson of Aalborg Amt.  She has a handful of matches in the FTDNA and MitoSearch databases.

Subclade K1c2-SWE
This individual's earliest known matrilineal ancestor was Maren Christensdatter of Aalborg Amt.  He has one full match in the FTDNA database with an individual whose earliest matrilineal ancestor was from southernmost Sweden, in an area that was historically Denmark.  (His match in the MitoSearch database is the same person.)  His markers match a variety of subclade K1c2 found in Sweden, as defined by William Hurst (administrator of the Haplogroup K project).

Clan Tara originated in the Near East some 45,000 years ago and entered Europe about 10,000 years ago where today it has an overall frequency of about 9%.  Originally concentrated in northwestern Italy, it is now more numerous along the Mediterranean and Altantic coasts of Europe and western Britain and Ireland.

Subclade T1
This individual descends from Ane Olesen of Allerslev, Præstø Amt.  His haplotype is rare, with no full HVR1+HVR2 matches in either the FTDNA or MitoSearch databases.

Subclade T2
This individual descends from Ane Marie Rasmusdatter of Næstved, Præstø Amt.  Her haplotype is rare, with no matches in either the FTDNA or MitoSearch databases.

Subclade T5
Subclade T5 is rare, with its greatest frequency (just 1.2%) in Norway. This individual's earliest known matrilineal ancestor is Maren Madsdatter of Hjørring Amt.  He has no matches in the FTDNA database.

In the trivia department:  Tsar Nicholas II of Russia was Hg T, through his maternal grandmother, Louisa, Queen of Denmark.

Haplogroup U is the oldest haplogroup in Europe, arriving before the Last Glacial Maximum and encountering, if not extinguishing, the Neanderthals.  They are today about 11% of the European population and are most common in western Britain and Scandinavia.

Subclade U2
We have one member who is subclade U2, a descendant of Karen Pedersdatter of Vejle Amt. 

Subclade U4
We have one member who is subclade U4, a descendant of Karen Nielsdatter of Præstø Amt.

Subclade U5b
We have one member who is subclade U5b, a descendant of Ingeborg BERTELSEN of Haderslev Amt.

Clan Velda is believed to have evolved from Clan Helena around 16-17,000 years ago in northern Iberia, a refugium from the Scandinavian Ice Sheet (SIS) during the last glacial maximum.  It is the youngest and rarest of European mtDNA haplogroups. Today, the haplogroup is found thinly across Europe, reaching its highest concentrations among the Basques of northern Spain and the Saami people of Finland — apparently, some of the clan advanced northward with the retreat of the SIS. 

One of our two Hg V members has an earliest known ancestor of Kirsten Olisdatter of Holbæk Amt; she has hundreds of matches with HVR1, but only one match with her combined HVR1+HVR2.  The other has an earliest matrilineal ancestor of Anne Kirstine Rasmusdatter of Odense Amt; his HVR1 is also common, with hundreds of matches, but his combined HVR1+HVR2 results are also common, with dozens of matches.  Still, our two Hg V members do not match each other.